Radar was developed in World War II to detect enemy aircraft. It has subsequently been refined to detect a large variety of objects, including ships, helicopters, satellites, and land vehicles. Radar systems typically work on the principle of bouncing microwave radiation off an object and analyzing the reflected signal (echo). The reflected signal can be used to develop information about the object, e.g., by measuring the round trip signal (echo) time the distance to the object can be determined, and by determining a Doppler shift of the echo a velocity of the object may be estimated. With sophisticated system design, object shape and size may be analyzed. Finally, a temporal analysis of sequential echoes may be used to detect a motion vector and characteristics of moving parts of the object.
Radar systems thus emit an electromagnetic wave or pulse, and analyze a reflection pattern to determine a characteristic of the object, distinguishing the object from clutter and background based on a return time of the echo. Radar is typically employed to determine an object profile, or shape, distance (range) and/or its velocity.
Electromagnetic detection techniques have developed to include radio frequency and microwave Radar, laser Radar (Lidar), FLIR (forward looking infrared), fluorescence techniques (especially to detect jet engine combustion emissions) and visual techniques.
Once Radar saw widespread use, military planners saw the military advantage that would accrue from having craft that are invisible to Radar. After decades of research and development, the United States began deployment of so called stealth aircraft in the 1980's. The next generation of helicopters, ships and missiles are also designed to be “stealthy”. Any craft can be designed to be stealthy, including land craft and satellites. The design principles of stealth aircraft are principally to (a) reduce radar reflections, especially retroreflections, of all kinds, and (b) to particularly hide or reduce characteristic signatures of aircraft, such as signals produced by engine turbine blades and wing surfaces.
The primary method for making an object stealthy is to reduce its radar cross section. While much of the performance of stealth aircraft is classified, it has been claimed a stealth fighter has the radar cross section of a normal plane the size of a bird, i.e., that it reflects no more microwave energy back to the detection device than a non-stealth plane the size of a bird.
There are several techniques, applied together, that are used to effect stealth. One is to design the craft so that flat surfaces are divided into small areas of various inclinations, disposed to avoid reflecting the signal directly back to the receiver. The craft is designed such that exposed surfaces present oblique angles or rounded surfaces that scatter or diffuse the signal away from the source of the radar beam. In addition, surfaces are coated with materials that absorb microwave radiation, and honeycomb sections are formed which trap microwaves, preventing reflections. See, U.S. Pat. Nos. 5,808,577, 5,697,394, 5,694,763, 5,536,910, 5,420,588, 5,276,477, 5,036,323, 4,606,848, 4,173,018, 4,117,485, 4,030,098, 4,019,699, expressly incorporated herein by reference.
The United States no longer has a monopoly on stealth craft. After the efficacy of stealth was proved during the Persian Gulf War, Germany, Russia, South Africa and other developed countries accelerated their development of stealth craft, and now not only have stealth craft for their own use, but are poised to sell stealth craft to developing countries, some of which are hostile to the United States.
Therefore, it is becoming increasing important for any military to be able to detect stealth craft.
The existing methods for detection of stealth aircraft include thermal signatures, exhaust signatures, acoustic signatures (see, U.S. Pat. No. 4,811,308, expressly incorporated herein by reference), radar employing non-traditional wavelengths (see, U.S. Pat. Nos. 5,850,285, and 5,657,022, expressly incorporated herein by reference), satellite imagery, and analysis of radio frequency emissions from the aircraft. However, none of these methods replaces traditional Radar monitoring.
U.S. Pat. No. 5,990,822 (Honigsbaum, Nov. 23, 1999), expressly incorporated herein by reference, describes a system for detecting stealthcraft based on silhouette methods, e.g., where the receiver aperture is aligned with an outgoing transmitted beam. Distance to an object is estimated by triangulation and sequence of beam returns.